It is known in producing resilient, compressible printing blankets to incorporate a cellular, foamed rubber intermediate layer. U.S. Pat. No. 3,887,750 shows the use of discrete hollow fibers to obtain a closed cell foam structure while U.S. Pat. No. 3,795,568 shows the use of particles of compressible latex foam rubber to obtain closed cell rubber structures. U.S. Pat. No. 4,025,658 discloses the production of a compressible printing blanket by adding and mixing particles of hydrated magnesium sulfate in the elastomeric matrix of the layer, creating a blowing effect, and leaching the particles from the matrix to produce a compressible layer having cavities which are interconnected by passages.
Printing blankets containing the compressible foam layers provide many advantages including most importantly, resistance to smash and operating latitude. However, these blankets have been found to be dimensionally unstable in that upon compression the foam layers tend to distort and flow in any direction which ruins print quality.
The use of a fabric layer between the compressible layer and the surface printing layer gives the blanket better dimensional stability than the blankets without such a stabilizing layer. For example, U.S. Pat. No. 4,174,244 shows such a fabric layer. Blankets containing the fabric layer however, suffer from a reduction in print sharpness and a phenomenon known as "falloff at the gap".
Falloff at the gap is a reduction in the blanket thickness in the area near the edges of the gap. This is caused by the longer path the upper fabric layer must follow as it is folded over into the gap for retention on the cylinder. The fabric, which is required to travel a greater distance in conforming to the gap's surface than the underlying foam layer, cannot elongate sufficiently and thus compresses the underlying foam layer. This reduction in the blanket's thickness near the gap causes a reduction in the printing pressure applied at that location, thereby reducing the amount of ink transferred at that point. The reduction in printing pressure causes print quality to suffer at that location. Many printers do not print at that location because it is so close to the edge of the finished page, but is a major deficiency when printers are attempting to produce pages printed over their entire length.
U.S. Pat. No. 4,303,721 discloses a blanket construction which contains a hard rubber stabilizing layer between the compressible cellular layer and the printing surface layer. The hard stabilizing rubber layer made possible the elimination of a woven stabilizing layer between the compressible layer and the printing surface layer and thus eliminated the problems of reduced print sharpness and falloff at the gap. The hard rubber layer between the compressible layer and the printing surface layer was described as having to have a durometer of between 75 and 95 (Shore A). It was indicated that generally such rubbers will contain substantial amounts of inorganic fillers or carbon black and more rigid thermosetting polymers such as phenolic resins to achieve this hardness.
Printing blankets prepared according to U.S. Pat. No. 4,303,721 have shown excellent performance on sheetfed presses with regard to print quality and register control, however, blankets prepared according to this technology have not performed as well on multiple color web presses because of poor register control (misalignment of colors) after paper splices or blanket washes. Furthermore, their performance is very sensitive to packing height. Packing height is defined as the height of the printing surface of the blanket (measured in thousandths of an inch) above the bearer height of the blanket cylinder. Blankets prepared according to U.S. Pat. No. 4,303,721 tend to exhibit poor packing latitude, i.e., must be packed to within plus/minus 0.001" of optimum height or they will result in poor register control (color movement), web wrinkles and web narrowing due to excessively high tensions between successive printing units. Blankets also exhibit the unusual property of feeding less web through the printing nip as packing heights are increased (negative web feed). By contrast, blankets which have a fabric layer above the compressible layer feed more web through the printing nip as their packing height is increased (positive web feed).
The present invention provides a compressible printing blanket having performance characteristics equal to or greater than a blanket containing a fabric but without the drawbacks of loss of print sharpnes or the phenomenon of falloff at the gap. Additionally, it provides a neutral web feed, i.e., tension on each side of the press nip is essentially equal as well as excellent packing latitude, thus overcoming the problems with blankets made in accordance with U.S. Pat. No. 4,303,721.